Design patterns

The Gang of Four patterns plus the enterprise patterns (Repository, Unit of Work, Dependency Injection, Specification, Pipeline, Event Bus) that LLD interviews actually reward. Every page gives the intent, a classDiagram whose names match a tested module in code/patterns/, the Pythonic variant that is often enough, the classic confusions and the LLD problems where the pattern earns its place. Start with the overview and the P0 patterns - Strategy, State, Observer, Factory Method, Builder, Decorator, Command - which cover most of what interviewers ask you to name and justify.

Tiers: P0 must be solid before any interview, P1 is expected of a strong SDE2, P2 is stretch material for when the rest is done.

Start here (P0)

• Design patterns overview
• Singleton
• Factory Method
• Builder
• Adapter
• Decorator
• Facade
• Command
• Observer
• State
• Strategy
• Template Method
• Repository
• Dependency Injection

All pages

#	Page	Tier	Covers
1	Design patterns overview	P0	Catalogue table of all patterns (intent, one-line Python idiom); symptom-to-pattern guide; anti-patterns (pattern-itis); the classic…
2	Singleton	P0	Intent: one instance, global access point. Example: thread-safe ConfigManager via new + lock (double-checked).
3	Factory Method	P0	Intent: defer concrete class choice to a creator. Example: NotificationSenderFactory.create(channel).
4	Abstract Factory	P1	Intent: families of related objects. Example: CloudProviderFactory producing (Storage, Queue) for AWS or GCP.
5	Builder	P0	Intent: stepwise construction with validation. Example: fluent MeetingBuilder or HttpRequestBuilder with build() validation.
6	Prototype	P2	Intent: clone existing objects. Example: Board.clone() for game-tree search.
7	Adapter	P0	Intent: convert an interface into the one clients expect. Example: StripeAdapter(PaymentProcessor) wrapping a vendor SDK stub.
8	Bridge	P2	Intent: decouple abstraction from implementation so both vary.
9	Composite	P1	Intent: tree of part/whole treated uniformly. Example: FileSystemNode (File, Directory) with size().
10	Decorator	P0	Intent: add responsibilities dynamically. Example: Beverage add-ons; RetryingSender(Sender).
11	Facade	P0	Intent: simple entry point to a subsystem. Example: CheckoutFacade.place_order().
12	Flyweight	P2	Intent: share intrinsic state across many objects. Example: Glyph/Style cache in an editor; shared chess piece types.
13	Proxy	P1	Intent: control access to an object (lazy, caching, protection, remote).
14	Chain of Responsibility	P1	Intent: pass a request along handlers until one handles it. Example: ATM cash dispenser (2000, 500, 100 notes).
15	Command	P0	Intent: request as an object for undo, queueing and logging.
16	Interpreter	P2	Intent: a grammar plus an evaluator. Example: rule engine for expressions such as amount greater than 100 AND country equals US.
17	Iterator	P1	Intent: sequential access without exposing structure. Example: DFS/BFS iterator over a file tree; paginated iterator.
18	Mediator	P1	Intent: centralize many-to-many interactions. Example: ElevatorController, ChatRoom.
19	Memento	P1	Intent: snapshot and restore without breaking encapsulation.
20	Observer	P0	Intent: one-to-many change notification. Example: PriceFeed notifying watchlists and alerts.
21	State	P0	Intent: behaviour varies by state with explicit transitions.
22	Strategy	P0	Intent: interchangeable algorithms behind one interface. Example: PricingStrategy (hourly, flat, daily cap).
23	Template Method	P0	Intent: algorithm skeleton with overridable steps. Example: BoardGame.play() shared by tic-tac-toe and snake-and-ladder.
24	Visitor	P2	Intent: add operations over an object structure without changing it.
25	Repository	P0	Intent: collection-like persistence abstraction. Example: UserRepository Protocol with in-memory and sqlite3 implementations.
26	Unit of Work	P1	Intent: track changes and commit atomically. Example: UnitOfWork context manager over repositories (sqlite3).
27	Dependency Injection	P0	Intent: supply collaborators from outside. Example: OrderService(repo, payments, clock, notifier) with fakes in tests.
28	Null Object	P1	Intent: a do-nothing implementation replaces None checks. Example: NullNotifier, NoDiscount.
29	Specification	P1	Intent: composable business rules combined with and, or and not.
30	Object Pool	P1	Intent: reuse expensive objects. Example: ConnectionPool with queue.Queue, acquire/release, health check.
31	Pipeline and Middleware	P1	Intent: ordered stages that transform or short-circuit a request.
32	Event Bus	P1	Intent: in-process pub/sub by topic, sync or async. Example: EventBus.subscribe(topic, handler), publish(event), worker-thread dispatch…